Should we relax and learn to love the budget deficit? Bankers yelp and businessmen tremble at the size of the president's river of red ink. Congressmen swarm like bees, in desperate search for a safer hive above the waterline.

Yet down the river swims the president, serenely. It was Reagan who helped teach Americans to fear and loathe a budget deficit, as if it were the root of all evil. Now, he says, it's a "necessary evil." The president has become the nation's most prominent and influential convert to the faith that budget deficits are overrated.

Lyndon Johnson was the last president to produce a fiscal surplus--$3.2 billion in the final budget of his administration. Dwight Eisenhower balanced the 1960 budget, by a bare $300 million. All the other years were in the red, and yet we managed to survive and prosper. Is it mere political carping to want the budget deficit down?

Most bankers and businessmen don't think so. The prestigious Business Roundtable blames the president's deficits for the continuing level of high, inflation-adjusted interest rates, which is knocking off a good chunk of American business.

"Without a sharp drop in interest rates there can be no reasonable recovery before the fourth quarter," a Roundtable spokesman says.

So who is right? The president, who swims serenely, or the would-be rescuers shouting from the riverbanks? I hold with the rescuers, for these reasons:

* This deficit is not being financed in the old, easy-money way.

It used to be that when the government spent more than it took in, it printed whatever money was needed to make up the difference. That helped produce our high inflation.

So now, the government borrows the extra money it needs from the private economy, and a huge deficit means a lot of borrowing. Interest rates can fall in the face of big government borrowing, as is happening now, as long as the economy remains in recession.

But when the economy starts to recover later this year, business will need more funds for expansion. At that point, government and business will clash. "Public and private borrowers, competing for the same funds, will drive up interest rates and dump the economy back into recession next year," says Irwin Kellner, chief economist at Manufacturers Hanover Trust in New York. "The only way to avoid that is to lower government borrowing needs by getting the deficit down."

Administration officials disagree. They say that there will be enough new savings in the economy to finance both the big budget deficits and a healthy business recovery--an opinion widely disputed by bankers.

Kellner has calculated that back in 1960, total federal financing took 20 percent of the savings in the economy, leaving 80 percent for business. This year, with personal savings running at the same rate it did in 1960--government will take 90 percent of the savings, leaving only 10 percent for business.

As long as there is a slow-growth money policy to fight inflation, a big deficit acts like a mighty magnet, pulling funds out of the private economy. That's bad for business.

* Supply-side economics won't save us. The president hopes that his tax-cut policies will generate enough economic activity to raise tax collections and narrow the deficits. But his own budget projections belie his theory. According to the budget, the 1981 tax-cut law will reduce government receipts by $38 billion this year, $92 billion in 1983, $139 billion in 1984 and $177 billion in 1985. The revenue drains get increasingly worse.

* It frightens the financial markets to see a president quit trying to balance his budget. All past presidents predicted budget surpluses, even though they failed to reach them. Reagan is the first to predict deficits as far ahead as anyone cares to look.

His budget also projects historically high real interest rates over the next three years--exactly what business is most afraid of. After subtracting inflation, Treasury-bill returns averaged 1.5 percent in 1960-69 and 3.7 percent in 1980-81. They are expected to average 4.1 percent from 1982 to 1985, according to the president's budget. That's a high real-interest-rate burden.

Bankers are keeping some big companies afloat that are technically in default, but how long can they continue to do so? Some straws in the wind suggest that the economy might be bottoming out and preparing to recover. "But if the deficit keeps on soaking up capital, the recovery won't last long," Kellner says flatly.

The president should change his mind about his deficits. It's too bad that Jimmy Carter gave mind-changing such a bad name.